Fully exposed Pt clusters for efficient catalysis of multi-step hydrogenation reactions

For di-nitroaromatics hydrogenation, it is a challenge to achieve the multi-step hydrogenation with high activity and selectivity due to the complexity of the process involving two nitro groups. Consequently, many precious metal catalysts suffer from low activity for this multi-step hydrogenation reaction. Herein, we employ a fully exposed Pt clusters catalyst consisting of an average of four Pt atoms on nanodiamond@graphene (Ptn/ND@G), demonstrating excellent catalytic performance for the multi-step hydrogenation of 2,4-dinitrotoluene. The TOF (40647 h−1) of Ptn/ND@G is significantly superior to that of single Pt atoms catalyst, Pt nanoparticles catalyst, and even all the known catalysts. Density functional theory calculations and absorption experiments reveal that the synergetic interaction between the multiple active sites of Ptn/ND@G facilitate the co-adsorption/activation of reactants and H2, as well as the desorption of intermediates/products, which is the key for the higher catalytic activity than single Pt atoms catalyst and Pt nanoparticles catalyst.

2. The conclusion that the cluster is only single-atomic-layer thick, derived from the STEM investigation (Fig. 1e).However, the model of the Pt4 cluster is standing upright and would be higher than a monolayer.Is this in agreement with the experimental observations?3. The author should carefully check the figure legends of Fig. 2g, and all other figures should be double-checked.4. In Fig. 3, the details for the amount of Pt to substrate and reaction conditions should be explicitly provided in the figure captions to allow the reader to qualify the results.5.The paper states that the fully exposed Pt clusters catalyst provide excellent reactivity compared with other catalytic systems.However, no data is presented on a commercial catalyst.The authors should collect their own data under same experimental conditions as a control.6.The authors' experimental results demonstrate that the performance of Ptn/ND@G is stable in a 5cycle test with no significant loss of activity.I wonder whether the catalyst has any leaching, ICP of the hot-filtration experiment should be provided.7. The author should include more comments (in the SI) on side products for low-selectivity catalysts (e.g., Pt/G or Pt/ND), which is helpful for understanding different mechanism between these catalysts.
Reviewer #3 (Remarks to the Author): The presented paper focuses on the hydrogenation of nitro-aromatics catalyzed by Pt and highlights the high activity of small clusters compared to single-atom and nanoparticles.The manuscript is wellorganized and easy to follow.The written expression is outstanding; only a misspelt in line 167 was found "beclearly", i.e., lack of space.Some statements need to be clarified before publication.The repeated terminology "fully exposed" is not defined and is rather unclear and unprecise.These tiny clusters seem to have a flat morphology parallel to the support (line 105); therefore, half of the cluster is exposed to the reaction medium.What does "both" refer to in line 64?Some references are missing, for instance, backing the sentence in line 72.The activity is described in lines 113 and 114, but for which process?In lines 146-147, the lack of Pt diffraction peaks is associated with high dispersion, but it is actually due to the small size of these clusters.A more critical issue is the lack of clarity in the neighbouring atoms to Pt, i.e.C or O.According to the positive oxidation state, Pt could be surrounded by oxygen; however, the argumentation is not pursued.Catalysts in line 231 are required to be accurately characterized or provide a reference to their characterization.In the DFT model, Pt presents negative charges which lack units.At the end of line 299, "electronic structure" should be Bader analysis; there is a considerable difference between these terms.The atomistic models are inconclusive: H2 should dissociate before reacting, and therefore, the co-adsorption energies are practically unrelated to the hydrogenation activity.Does the simulation consider longrange dispersion corrections?
As a result, we think that there is probably no H-transfer from Pt4 clusters to Pt single atoms in the present catalytic system.Table R1.The adsorption energies (Eads) of H* on Pt4@GV3, NGD@GV1, NGD@GV2, NGD@GV3, and Pt1@GV1 sites, respectively.
Eads(eV) unless the authors can demonstrate their existence on their carbon material (for exemple by EPR).Furthermore, for Pt1@C the CN does not correspond to the model presented and the presence of chlorine (CN=2) does not appear in the model.The proposed models seem really oversimplified to me to be used for rationalization.

Response:
We thank the reviewer for this nice comment.
(1)To address this concern, we performed Raman spectroscopy (Figure R2) to further characterize the degree of defect on ND@G, Pt1/ND@G, and Ptn/ND@G.The D band (-1400 cm -1 ) originates from the disordered carbon, whereas the G band (-1580 cm -1 ) is related to the vibration of sp 2 carbon atoms.The intensity ratio of D band to G band (ID/IG) indicates the degree of disorder and defects of carbon materials.These results indicate that all the ND@G, Pt1/ND@G, and Ptn/ND@G have rich carbon defective sites.In addition, the literature has also proved that the monovacancy can exist stably and serve as an adapted host for trapping metal atoms.(J.Am.Chem. Soc. 2022, 144, 2171-2178.)(2)In fact, the effect of a small amount of Cl coordination on the Pt single atom sites was ignored in the construction of the DFT model (ACS Catal. 2022, 12, 8104-8115)and we drew on this idea when building the model in the original manuscript.
Herein, we constructed the Pt1Cl2@GV2 model according to the coordination structure from experimental results (CN(Pt-Cl) = 2.1, CN(Pt-C) = 1.7), and calculated the adsorption energies of the intermediates in the hydrogenation of 2,4-DNT to make sure the reliability of our conclusion.As shown in Table R3, due to the steric hindrance on single Pt atom, the very weak adsorption of reactants (-0.09 eV) and intermediates (-0.09, -0.13 and -0.22 eV) would occur on the Pt1Cl2@GV2.As a contrast, the adsorption of 2,4-DNT on Pt1@GV2 is stronger than on Pt4 clusters and Pt(111) surface ( -0.45 vs. -0.27 and -0.23 eV), consistent with our previous discussion on Pt1@GV1.Therefore, although Pt-Cl bond is detected in our single-atom catalyst, the working active site is Pt metal atom only coordinated with C atom.Our models of Pt1@GV2 and Pt1@GV1 are also reasonable for explaining the experiment.

Figure R2
. Raman spectra of ND@G, Pt1/ND@G, and Ptn/ND@G.this does not allow us to conclude as to the possible stability of the catalyst.

Response:
We thank the reviewer for this constructive suggestion.We add 5 recycling tests at low conversion (Figure R3).The HAADF-STEM and ICP analyses also applied to characterize the used catalyst after the 5th cycle.The results and discussion were shown in the revised manuscript."The Ptn/ND@G can be used for 5 cycles at low conversion without any loss in activity (Supplementary Fig. 8).The STEM images and ICP results of Ptn/ND@G after the reaction indicated the absence of any aggregation and leaching, demonstrating its outstanding structural stability during the reaction (Supplementary Fig. 9 and Supplementary Table 4)."(Please see Line 267-271) . Recycling test for 2,4-DNT hydrogenation over the Ptn/ND@G.
Comments 7. The final part of the article aimed at explaining the difference in performance between isolated atoms and clusters is of little interest.It is well known in the literature that isolated atoms do not perform well for these reactions, and this is discussed in the introduction to the article.On the other hand, it would make sense to try to explain the difference in TOF observed between the clusters and the Pt particles.
It is on this last point that the authors must concentrate.

Response:
We thank the reviewer for the nice suggestion.Following the reviewer's suggestion, we add the DFT calculations and discuss the difference of catalytic performance between the clusters and the Pt particles.Although the easier H2 dissociation is found on Pt(111) (Table R4), the adsorption of the product 2,4-DAT are much stronger than Pt4 clusters (Table R5).Consequently, the product would preferentially adsorb and accumulate on Pt(111), leading to a declined activity.Above results and discussion are shown in the revised manuscript.We add and rewrite "The hydrogenation of 2,4-DNT is a super-exothermic reaction, which is verified experimentally and theoretically.And our DFT calculations also show the same result (Supplementary Table 5).This leads to the fact that once the reactants can be adsorbed onto the active site, it will be a key factor for the reaction activity that whether the product and/or intermediates can desorb and release the active site successfully.To understand the difference for the hydrogenation on Pt single atoms, Pt clusters, and Pt metal particles.Based on the results of XAFS and STEM, we used Pt1@Gr, Pt4@Gr and Pt(111) (Supplementary Fig. 10) as the models, respectively."(Please see Line 302-309); "On Pt(111) surface, the adsorption by 2-NO2-, 4-NO2-, and benzene-has the adsorption energies of -0.18, -0.24, and -0.23 eV, respectively."(Please see Line 315-317); "On Pt(111) surface, the dissociation of H2 is spontaneous, which is exothermic by 1.13 eV, implying that a potentially high catalytic activity of 2,4-DNT."(Please see Line 333-335); "Compared Pt(111) with Pt4@Gr, it is found that although easier H2 dissociation (-1.13 eV) on Pt(111), the adsorption of the product 2,4-DAT is too strong (Supplementary Table 7) to desorb, which makes the product accumulation on active sites and causes a decrease in activity." (Please see Line 339-342) In this manuscript, the authors fabricated a fully exposed-Pt-cluster catalyst, consisting of an average of four Pt atoms supported on a defective nano-diamond graphene (Ptn/ND@G), for the complex multi-step hydrogenation reactions of di-nitroaromatics.
The physicochemical properties of the catalysts were thoroughly investigated using HAADF-STEM, XAFS, DRIFTS and DFT, which provided compelling support for the proposed reaction mechanism.The robust catalytic performance and good stability for the hydrogenation of 2,4-dinitrotoluene under mild reaction conditions outperform other known catalysts.This work offers a new application for the multi-step hydrogenation into the fine chemical sector under mild conditions.Overall, the manuscript reports a nice and systematic work, with a logical methodological approach and satisfactory discussion in the context of literature data.I recommend acceptance of this manuscript in Nature Communications after minor revision, comments are shown below: Comments 1.The nanodiamond@graphene hybrid carbon material (ND@G) is an intriguing system.A brief discussion explaining why the ND@G support could highly disperse the metal atoms would be helpful.

Response:
We thank the reviewer for this nice comment.Strong electron transfer between Pt and the rich structural defects in the graphene shell of ND@G results in strong metal−support interactions (SMSIs).These interactions are energetically favorable for anchoring metal atoms and stabilizing atomically dispersed metal species through metal-C bonding.
Comments 2. The conclusion that the cluster is only single-atomic-layer thick, derived from the STEM investigation (Fig. 1e).However, the model of the Pt4 cluster is standing upright and would be higher than a monolayer.Is this in agreement with the experimental observations?
Response: We thank the reviewer for the nice suggestion.We are very sorry for not  Response: We thank the reviewer for pointing this out for us.Following the suggestion, we have added the experimental results and corresponding description of commercial Pt/C catalyst under same experimental conditions as a control in the revised manuscript.
The results show that the fully exposed Pt cluster catalyst provides the excellent reactivity compared with other catalytic systems and commercial Pt/C catalyst.We add "Furthermore, the commercial 5 wt% Pt/C (characterized by STEM, Supplementary Fig. 4) were used as reference samples.As shown in Figure 3c, compare with Ptn/ND@G, the commercial Pt/C catalyst displayed low activity."(Please see Line 238-240) Comments 6.The authors' experimental results demonstrate that the performance of Ptn/ND@G is stable in a 5-cycle test with no significant loss of activity.I wonder whether the catalyst has any leaching, ICP of the hot-filtration experiment should be provided.

Response:
We thank a lot for your careful review.In the revised manuscript, we add the experiments and characterization of 5 recycling tests at low conversion (Our answer to the Reviewer #1, Comments 6).We add "The Ptn/ND@G could be used for 5 cycles at low conversion without any loss in activity as shown in Supplementary Fig. 8.The STEM images and ICP results of Ptn/ND@G after the reaction indicated the absence of any aggregation and leaching, demonstrating its outstanding structural stability during the reaction (Supplementary Fig. 9 and Supplementary Table 4)."(Please see Line 267-

271)
Comments 7. The author should include more comments (in the SI) on side products for low-selectivity catalysts (e.g., Pt/G or Pt/ND), which is helpful for understanding different mechanism between these catalysts.

Response:
We appreciate the reviewer for the suggestion.In Figure R5

Reviewer #3 (Remarks to the Author):
The presented paper focuses on the hydrogenation of nitro-aromatics catalyzed by Pt and highlights the high activity of small clusters compared to single-atom and nanoparticles.The manuscript is well-organized and easy to follow.
Comments 1.The written expression is outstanding; only a misspelt in line 167 was found "beclearly", i.e., lack of space.Some statements need to be clarified before publication.

Response:
We wholeheartedly thank the reviewers for spotting the mistakes in our original submission.In the revised manuscript, we have carefully checked the text and corrected the typos.(Please see Line 173) Comments 2. The repeated terminology "fully exposed" is not defined and is rather unclear and unprecise.These tiny clusters seem to have a flat morphology parallel to the support (line 105); therefore, half of the cluster is exposed to the reaction medium.

Response:
We thank the reviewer for the comment which inspired us to add clearer and more precise discussion of the "fully exposed"."Fully exposed" means that all metal atoms on the catalyst are fully exposed on the surface, and there is no bulk phase structure.The fully exposed structure well-guarantee the maximum atomic utilization during the reaction.Hence, the fully exposed cluster catalyst can not only provide the multiple metal sites but also maintain a full atomic utilization efficiency.FECC is so highly dispersed that all the metal atoms within it are available for the adsorption and transformation of reactants.Following the reviewer's advice, we added more detailed discussion in the revised manuscript."As a cross-dimensional extension to the concept of SACs, fully exposed cluster catalysts (FECCs) emerge as a type of catalyst which all metal atoms on the catalyst are fully exposed on the surface of the active sites without bulk phase structure.This design guarantees full atomic utilization during the reaction, maintaining a full atomic utilization efficiency and providing catalytic sites with multiple metal atoms.FECC is so highly dispersed, allowing all the metal atoms to be available for the adsorption and transformation of reactants.More importantly, supported FECCs exhibit two distinct features.The primary advantage lies in its ultrasmall size (normally below 1 nm), which eliminates the presence of undesired bulk atoms and reduces the average coordination number of the metal atoms.Another advantage is a small contact angle between the metal and support, forming a layered structure, which enhances the interaction between metal atoms and support, and ultimately increasing the stability of the clusters.From this viewpoint, FECCs with multiple active sites have distinct advantages in overcoming the limitations of SACs for a multi-step reaction."(Please see Line 94-170) And the detailed description of fully exposed clusters could be followed in our review article (ACS Cent. Sci. 2021, 7, 262−273).
Comments 3. What does "both" refer to in line 64?
Response: We appreciate the reviewer for his/her carefulness.In this sentence, the word "both reacting species" refer to H2 and 2,4-DNT.We are sorry that our ambiguous writing brings much confusion to the reviewer.Here we added and modified the sentence with reviewers' comments."Density functional theory (DFT) calculations and absorption experiments reveal that the fully exposed Pt clusters with multiple metallic active sites benefit sufficient sites for co-adsorption reactants and H2, the subsequent dissociation of H2, and the release of active sites due to moderate adsorption ability of intermediates and products, leading to enhanced catalytic performance."(Please see (2) The results included van der Waals corrections (PBE+D3+ZPE) (Table R6) are used to evaluate long-range dispersion corrections.It is found that i) the adsorption energies (E1) of 2,4-DNT are over-corrected (−0.23 vs. −1.41eV; −0.27 vs. −1.39eV; −0.53 vs. −1.71 eV) on Pt(111), Pt4 cluster and Pt single atoms; ii) difficult dissociation of H2 (0.47 eV) on Pt single atoms after adsorbing 2,4-DNT, which causing the low catalytic activity.Therefore, the same trends and conclusions are obtained for PBE+D3+ZPE and PBE+ZPE.Due to the lack of accurate experimental values as a reliable reference, the current method (PBE+ZPE) is reasonable for comparing reactivity.
Table R6.The calculated reaction energies (E1, E2, E3 unit: eV) of R1, R2, R3 on Pt(111), Pt4@Gr, Pt1@Gr catalysts with PBE+ZPE and PBE+D3+ZPE.The authors have satisfactory answer to most of my comments.Before possible submission I still have some concerns considering metal single atoms coordination and possible H-spillover.
Considering the preparation route of this material (high temperature heat-treatment under inert and then air exposure at room temperature, the material should contain significant amount of surface oxygen functional groups.And this has indeed been reported by the authors in 10.1038/s41467-019-12460-7 (fig S4) and https://doi.org/10.1021/jacs.8b07476(figure s2).The presence of these groups should be at least commented, but also considered when discussing single atom coordination (XAS does not make the distinction between carbon and oxygen).
The presence of these groups should also be considered for H-spillover, as different works have shown that these groups are the shuttles for H-spillover.Thus these groups should be considered in the DFT calculations (Tables R1 and R2).Moreover, simple experiments (maybe shorter than calculations) with WO3 can show the presence or not of H-spillover.
Reviewer #2 (Remarks to the Author): All my comments have been addressed in a professional and convincing manner.
Reviewer #3 (Remarks to the Author): The submitted reviewers' rebuttal does not answer the reviewers' questions and comments.Besides, the answers to the Reviewers should be integrated into the manuscript wherever suitable.
Reviewer#1Q1 does not ask for the properties of the ND@G material but for the label ND@G.
In Reviewer#1Q3 and Q5, the authors made a significant number of extra calculations but did not address the reviewer's question.It has been shown that C-vacancies anchor H atoms (DOI: 10.1021/acs.jpcc.1c03996).It is evident that CH3OH will not take another H to transfer it.However, CH3O-could get the H from Pt4 and bring it to Pt1, which has not been considered.Why is the energy of H*(at C far from Pt4 site) and H*(at C far from Pt SP site) so different (1.34 vs 86 eV)?Statistical analysis would reveal the H's probability of moving from Pt4 to the C surface to Pt1, which is ~0.3 eV more stable and thermodynamically driven.
Reviwer#1Q4.(1) Raman spec will also reveal vacancies in the particle core, meaning the resulting evaluation is unsuitable.
(1) The Pt1Cl2 should be compared with the Pt4Clx model, as the Pt4 coordination also indicates monovacancy; the Eads difference in the presence of Cl is significant.
Reviewer#1Q7.The Eads value for 2,4-DAT is significantly larger than the intermediates on Pt(111), which is strange considering that 2,4-DNT adsorption is weaker than on Pt4.There is no schematic representation or further details of these surface structures.Being a crucial part of the discussion, these structures should be somewhere in the paper, SI preferentially.
Answers regarding nanoparticles in #1Q7 and commercial nanoparticles in #2Q5 should be interpreted and critically discussed in the manuscript.
Reviewer#3Q2's answer says, "Another advantage is a small contact angle between the metal and support, forming a layered structure, which enhances the interaction between metal atoms and support, and ultimately increases the stability of the clusters."If the structures are layered, the DFT model for Pt4 is wrong.Layered structures are different from FECC.
Reviewer#3Q9.Since when is eV the unit of charge?
Reviewer#3Q10. (1) Based on H2 adsorption and dissociation energies and upon flushing the reactor 3 times with H2, the catalyst will inevitably have H-adatoms on the surface.If the catalysts were saturated with 2,4-DNT, as said in the response, there would be no sites for H2 to dissociate, so the reaction would not proceed.( 2) What does it mean that the adsorption energies are over-corrected?How is it possible that long-range interaction increases the reaction energy (E1) from -0.23 to -1.41 eV compared with these on E2 and E3? -This question remains unanswered and, in agreement with #3, it is critical for the evaluation of the models.

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): Comment.s2).The presence of these groups should be at least commented, but also considered when discussing single atom coordination (XAS does not make the distinction between carbon and oxygen).The presence of these groups should also be considered for Hspillover, as different works have shown that these groups are the shuttles for Hspillover.Thus, these groups should be considered in the DFT calculations (Tables R1   and R2).Moreover, simple experiments (maybe shorter than calculations) with WO3 can show the presence or not of H-spillover.

Response:
We thank the reviewer and appreciate the insightful comment and suggestion, which has helped us much in improving the quality of our manuscript.
Although we have satisfactory answer to most of his/her comments, we are still sorry that some points are not well addressed in the 1 st -round response.We would like to address this comment from the following aspects.
(1) As the reviewer mentioned, our previous paper has reported that the ND@G inevitably contain some surface oxygen functional groups.XPS spectra of the ND@G confirmed that the oxygen functional groups including C-O (oxygen singly bonded to aliphatic carbon), C=O (oxygen doubly bonded to aromatic carbon), and a small number of phenolic (oxygen singly bonded to aromatic carbon).In our study, Pt atoms were loaded onto the ND@G surface and subsequently subjected them to reduction in H2 at 200 ºC.Following the reduction treatment, a significant decrease in oxygen content was observed (from about 3.9% down to approximately 2.5%), indicating a remarkable reduction in the content of the relevant oxygen functional groups.We agree with the reviewer that the oxygen functional group could potentially serve as the shuttles for H-spillover.However, due to the limited presence of oxygen functional groups in our research system, the feasibility of hydrogen transfer through these groups is diminished.
(2) According to the fitting results of EXAFS, due to the difficulty in distinguishing between O and C atoms in EXAFS, it is speculated that the structure may include oxygen coordinated structures, namely Pt1O2@Gr and Pt1O1C1@Gr.We calculated the energies of possible structures of Pt1O2@Gr (Table R1) and Pt1O1C1@Gr (Table R2).As a result, it was found that Pt1O2@Gr and Pt1O1C1@Gr has a possible stable structure of Pt1O2@Gr-(2) and Pt1O1C1@Gr-( 6) respectively.Considering the reducing atmosphere in the reaction (25 ℃, 1 MPa H2), the oxygen in the oxygencontaining structure is likely to be reduced by H2 to form H2O. Therefore, we considered the thermodynamic possibilities of the following reactions.
(3) In response to the reviewer's suggestion, we performed the WO3 experiments (Figure R1).The inherent color of pure WO3 is canary yellow, whereas the combination of catalysts and WO3 exhibits a dark yellow.The color of WO3 remained unchanged in the presence of H2.After exposing the mixture of Ptn/ND@G and WO3 to the H2 atmosphere, no significant hydrogen spillover phenomenon was observed.
In summary, in our research system, we propose that Pt4 clusters play a major role in the hydrogenation process of 2,4-DNT.H2 dissociates at the Pt4 sites where the reactants are adsorbed, preferring to react directly with 2,4-DNT at the Pt4 sites, enhancing the overall efficiency of the reaction.
explaining this question clearly.The claim of monolayered Pt clusters from the STEM investigation means that there are no Pt atoms covering each other and all Pt atoms in the clusters can be exposed to reactants during reaction.As illustrated in the top and side views of the DFT-calculated model in Figure R4 (a) and (b), the computed Pt4 cluster has no Pt-Pt overlay in the thickness direction and thus contains single Pt layer.

Figure
Figure R1.Photographs of samples made of WO3 mixed with the catalysts before The proposed coordination for the isolated Pt atom and the clusters involves a monovacancy.Such highly reactive species are not expected to survive the conditions used for catalyst preparation.This model does not seem justified to me,

Table R3 .
The adsorption energies Eads (eV) of intermediates on different single Pt atom models.

Table R4 .
The

Table R5 .
The adsorption energies Eads (eV) of intermediates on different catalysts.
The authors have satisfactory answer to most of my comments.Before

Table R1 .
The possible structures of Pt1O2@Gr.The energy (DFT calculation) is given to compare the relative stabilities of these structures.

Table R2 .
The possible structures of Pt1O1@Gr.The energy (DFT calculation) is given to compare the relative stabilities of these structures.

Table R3 .
The Gibbs free energy of species in reaction (1) and (2).